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Light and Sound - How shadows are formed and changed

Grade 3IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light travels in straight lines, which is known as the rectilinear propagation of light. When an object blocks these rays, a shadow is formed on the opposite side.

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A shadow is a dark area where light from a light source is blocked by an opaque or translucent object.

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Opaque objects block all light and create dark, well-defined shadows. Translucent objects block some light, creating faint shadows. Transparent objects block almost no light and usually do not create shadows.

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The size of a shadow changes depending on the distance between the light source and the object. If the distance dd between the source and the object decreases, the shadow size increases.

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The shape of a shadow can change if the object is rotated or if the angle of the light source changes. For example, shadows are longest when the Sun is at a low angle (morning/evening) and shortest when the Sun is directly overhead at noon.

📐Formulae

Object closer to light source→Larger shadow\text{Object closer to light source} \rightarrow \text{Larger shadow}

Object further from light source→Smaller shadow\text{Object further from light source} \rightarrow \text{Smaller shadow}

Lower light angle→Longer shadow\text{Lower light angle} \rightarrow \text{Longer shadow}

💡Examples

Problem 1:

Sarah is using a torch to make a shadow of a 10 cm10\text{ cm} tall action figure on a wall. If she moves the torch closer to the action figure, what happens to the size of the shadow?

Solution:

The shadow will become larger.

Explanation:

When the light source (torch) moves closer to the object, the object blocks a larger angle of the light rays spreading out from the source. This results in a larger area of darkness on the wall.

Problem 2:

At what time of day would a vertical pole cast the shortest shadow: 9:00 AM9:00\text{ AM}, 12:00 PM12:00\text{ PM} (noon), or 4:00 PM4:00\text{ PM}?

Solution:

12:00 PM12:00\text{ PM} (noon).

Explanation:

At noon, the Sun is at its highest point in the sky. Because the light rays are hitting the object from directly above, the area of blocked light on the ground is at its minimum.

Problem 3:

Explain why a glass window does not cast a dark shadow like a wooden door does.

Solution:

The window is transparent, while the door is opaque.

Explanation:

A wooden door is opaque and blocks the light rays (Light↛ObjectLight \nrightarrow Object). A glass window is transparent, meaning light rays pass through it (Light→Object→LightLight \rightarrow Object \rightarrow Light), so no significant dark area is formed behind it.